John Calvin Commentary Genesis 21:23

"Now therefore swear unto me here by God that thou wilt not deal falsely with me, nor with my son, nor with my son`s son. But according to the kindness that I have done unto thee, thou shalt do unto me, and to the land wherein thou hast sojourned." — Genesis 21:23 (ASV)

That thou wilt not deal falsely with me. Literally, it is, ‘If you lie;’ for among the Hebrews, a defective form of speech is common in taking oaths, which is to be explained thus: ‘If you should break the promise given to me, we call upon God to sit as Judge between us and to show himself the avenger of perjury.’

But ‘to lie,’ some here interpret as dealing unjustly and fraudulently; others, as failing in the conditions of the covenant. I simply understand it as if it were said, ‘You will do nothing treacherously with me or with my descendants.’

Abimelech also enumerates his own acts of kindness, the more effectually to exhort Abraham to exercise good faith. For, since he had been humanely treated, Abimelech declares it would be an act of base ingratitude if he did not, in return, endeavor to repay the benefits he had received.

The Hebrew word חסד(chesed) means to deal gently or kindly with anyone. For Abimelech did not come to implore compassion from Abraham, but rather to assert his own royal authority, as will appear from the context.